I was just looking for such a thing today, but using markdown and internal to discourse. I’m working with my family on identifying people in photos, and I’d love to be able to create a topic with a jpg in the OP, and let people reply to discuss the photo and describe everything they know about it and the people in it. The OP could be a wiki that can be edited to add annotations assigned to specific places on the photo. These would appear as hotspots when you mouseover the photo that can be clicked on to read the annotation about that spot.
I am pretty sure this does not exist and am just spitballing here. Maybe it could be a new plugin inspired by Discourse footnote which also provides inline expansion.